Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to roller cone drill bits for drilling earth formations, and more specifically to roller cone drill bit designs.
2. Background Art
Roller cone rock bits and fixed cutter bits are commonly used in the oil and gas industry for drilling wells. FIG. 1 shows one example of a roller cone drill bit used in a conventional drilling system for drilling a wellbore in an earth formation. The drilling system includes a drilling rig (10) used to turn a drill string (12) which extends downward into a wellbore (14). Connected to the end of the drill string (12) is roller cone-type drill bit (20), shown in further detail in FIG. 2.
FIG. 2 shows a roller cone bit (20) that typically comprises a bit body (22) having an externally threaded connection at one end (24), and a plurality of roller cones (26) (usually three as shown) attached at the other end of the bit body (22) and able to rotate with respect to the bit body (22). Disposed on each of the cones (26) of the bit (20) are a plurality of cutting elements (28) typically arranged in rows about the surface of the cones (26). The cutting elements (28) may comprise tungsten carbide inserts, polycrystalline diamond compacts, or milled steel teeth.
Significant expense is involved in the design and manufacture of drill bits to produce drill bits with increased drilling efficiency and longevity. For more simple bit designs, such as fixed cutter bits, models have been developed and used to design and analyze bit configurations having optimally placed cutting elements, a more balanced distribution of force on the bit, and a more balanced distribution of wear on the cone. These force-balanced bits have been shown to be long lasting and effective in drilling earth formations.
Roller cone bits are more complex in design than fixed cutter bits, in that the cutting surfaces of the bit are disposed on roller cones. Each of the roller cones independently rotates relative to the rotation of the bit body about an axis oblique to the axis of the bit body. Because the roller cones rotate independent of each other, the rotational speed of each cone is likely different. In some configurations, the cutting elements on the drive row are located to drill the full diameter of the bit. In such cases, the drive row may be interchangeably referred to as the xe2x80x9cgage rowxe2x80x9d.
Adding to the complexity of roller cone bit designs, cutting elements disposed on the cones of the roller cone bit deform the earth formation by a combination of compressive fracturing and shearing. Additionally, most modern roller cone bit designs have cutting elements arranged on each cone so that cutting elements on adjacent cones intermesh between the adjacent cones, as shown for example in FIG. 3A and further detailed in U.S. Pat. No. 5,372,210 to Harrell. Intermeshing cutting elements on roller cone drill bits is desired to permit high insert protrusion to achieve competitive rates of penetration while preserving the longevity of the bit. However, intermeshing cutting elements on roller cone bits substantially constrain cutting element layout on the bit, thereby further complicating the designing of roller cone drill bits.
Because of the complexity of roller cone bit designs, accurate models of roller cone bits have not been widely developed or used to design roller cone bits. Instead, roller cone bits have been largely developed through trial and error. For example, if it has been shown that a prior art bit design leads to cutting elements on one cone of a bit being worn down faster that the cutting elements on another cone of the bit, a new bit design might be developed by simply adding more cutting elements to the faster worn cone in hopes of reducing wear on each of the cutting elements on that cone. This trial and error method of designing roller cone drill bits has led to roller cone bits with cutting elements unequally distributed between the cones. In some prior art bit designs, the unequal distribution of the number of cutting elements between the cones may result in an unequal distribution of force, strain, stress, and wear between the cones, which can lead to the premature failure of one of the cones. In other prior art bit designs, the unequal distribution of the number of cutting elements between the cones may result in an unequal distribution of contact with the formation between the cones or an unequal distribution of volume of formation cut between the cones.
One example of a prior art roller cone bit configuration considered effective in drilling wellbores is shown in FIGS. 3A-3B. In FIG. 3A, the profiles of each of the cutting elements on each cone are shown in relation to each other to show the intermeshing of the cutting elements between adjacent cones. FIG. 3A has three cones (110) a first cone (114), a second cone (116) and a third cone (118). A plurality of cutting elements (112) are on each of the cones (110). The first cone (114) has three rows of cutting elements (112): a first row (114a), a second row (114b), and a third row (114c). The second cone (116) has two rows of cutting elements (112): a first row (116a) and a second row (116b). The third cone (118) has two rows of cutting elements (112): a first row (118a) and a second row (118b).
FIG. 3B shows a section of a drill bit that comprises a bit body (100) and three roller cones (110) attached to the bit body (100) such that each roller cone (110) is able to rotate with respect to the bit body (100) about an axis oblique to the bit body (100). Disposed on each of the cones (110) is a plurality of cutting elements (112) for cutting into an earth formation. The cutting elements are arranged about the surface of each cone in generally circular, concentric rows arranged substantially perpendicular to the axis of rotation of the cone. In this example, the rows of cutting elements are arranged so that cutting elements on adjacent cones intermesh between the cones. In this example, the cutting elements (112) comprise milled steel teeth with hardface coating applied thereon.
Although not shown in the drawings, each roller cone (26 in FIG. 2) may be rotatably mounted on a cylindrical bearing journal (not shown) on the bit body (22 in FIG. 2), as is known in the art. As is also known in the art, bearings such as roller bearings, ball bearings, or sleeve bearings may be located between the roller cone (26 in FIG. 2) and the bearing journal (not shown) to provide the rotational mounting.
In FIG. 4A, the roller cones (26) are illustrated schematically as simple frustoconical figures. Each roller cone (26) has an axis of rotation (32) passing substantially through the center of the frustoconical figure. The central rotational axis (34) of the bit (20 in FIG. 2) is illustrated as point (34) in FIG. 4 (since FIG. 4A is taken from a view looking directly along the rotational axis of the bit). From FIG 4A, it can be seen that because of the offset of axes (32), none of the axes intersect axis (34) of the bit. In this flat projection, the intersection of the axes (32) forms an equilateral triangle. The amount of offset for a bit is the distance from axis (34) to the mid-point of any side of triangle. In the prior art, the amount of offset was typically less than about {fraction (1/32)} inch of offset per inch of bit diameter. It was believed that offsets greater than that amount would cause high wear of gage elements resulting in loss of rate of penetration.
FIG. 5A is a cone profile which is an overlay in a single plane of one-half of all of the three roller cones (26) to indicate the journal angle (36) of the bit. The journal angle (36) is the angle that the bearing journal axis, which coincides with the rotational axis (32) of the roller cone (26), makes with a plane normal to the bit rotational axis (34). In the prior art, the journal angle was typically greater than about 32.5xc2x0. It was believed that journal angles smaller than that amount could result in poor wear properties of the cutting element materials and breakage of the bit body (22).
U.S. Pat. No. 4,611,673, issued to Childers et al., discloses a roller cone drilling bit comprising a plurality of conical roller cutters having hard metal cutting elements thereon and being so positioned relative to each other that their rotational axes are offset from the rotational axis of the drill bit, and a drilling fluid nozzle system for directing a pressurized fluid stream across certain of the cutting elements and thereafter against the formation generally at the bottom of the wellbore so that when the drill bit is used in its most advantageous areas, such as the soft, medium-soft and plastic formations, the nozzle system prevents xe2x80x9cballing upxe2x80x9d of the cutters and greatly increases the drilling efficiency of the bit.
U.S. Pat. No. 4,741,406, issued to Deane et al., discloses a tri-cone drill bit comprising a plurality of conical roller cutters having hard metal cutting elements thereon and being so positioned relative to each other that their rotational axes are offset from the rotational axis of the drill bit, and a drilling fluid nozzle system for directing a pressurized fluid stream across certain of the cutting elements and thereafter against the formation generally at the bottom of the wellbore so that when the drill bit is used in its most advantageous areas, such as the soft, medium-soft and plastic formations, the nozzle system prevents xe2x80x9cballing upxe2x80x9d of the cutters and greatly increases the drilling efficiency of the bit. In one embodiment the drill bit body has vertically extending recessed portions formed at the junctures of the lugs to provide flow passageways for the upward flow of drilling fluid and entrained cuttings.
U.S. Pat. No. 5,311,958, issued to Isbell et al., discloses an earth-boring bit that is provided with three cutters, two of the three cutters are provided with heel disk cutting elements defined by a pair of generally oppositely facing disk surfaces that generally continuously converge to define a circumferential heel disk crest. One of the two cutters having heel disk elements is further provided with an inner disk cutting element.
U.S. Pat. No. 6,095,262, issued to Chen, discloses a roller cone drill bit for drilling through subterranean formations having an upper connection for attachment to a drill string, and a plurality cutting structures rotatably mounted on arms extending downward from the connection. A number of teeth are located in generally concentric rows on each cutting structure. The actual trajectory by which the teeth engage the formation is mathematically determined. A straight-line trajectory is calculated based on the actual trajectory. The teeth are positioned in the cutting structures such that each tooth having a designed engagement surface is oriented perpendicular to the calculated straightline trajectory.
U.S. Pat. No. 6,213,225, issued to Chen, discloses a roller cone drilling wherein the bit optimization process equalizes the downforce (axial force) for the cones. Bit performance is enhanced by equalizing downforce.
One aspect of the invention is a roller cone drill bit for drilling earth formations and a method of using the same. The drill bit comprises a bit body and a plurality of roller cones attached to the bit body and able to rotate with respect to the bit body. The drill bit further comprises a plurality of cutting elements disposed on each of the roller cones. The bit has an offset of at least about 0.375 inches; and a journal angle less than about thirty degrees. In one embodiment, the drill bit comprises three roller cones. In another embodiment, the cutting elements of the bit are arranged on each cone so that cutting elements on adjacent cones intermesh between the cones.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.